If this would be the case, it wouldn't be a standard in modern JS frameworks. At least I can't think of any popular framework that doesn't use it, or a mix of PostCSS and PurgeCSS. Very big CSS Framework like Tailwind, Bootstrap and Bulma even recommend this approach unless you want to end up having CSS files with several megabytes.
I don't understand why you think you'd get lost in your CSS. Nothing changes at how or where you write your CSS. The only thing that changes is how much of your CSS you deliver to your users.
Loading another page also wouldn't be a problem since you could simply prefetch any missing CSS files or properties by hovering over a page link for example.